since you know x in terms of y, substitute in that value in place of x. so instead of x + y = 128, you ow have 3 + y + y = 128. combine like terms (y) to get 2y + 3 = 128. subtract 3 from both sides to get 2y = 125. then divide both sides by two to get y = 62.5. to find x, since you know x = 3 + y, just add 3 to 62.5 to get x = 65.5